Chengzhong Pan

Overland flow resistances on varying slope gradients and partitioning on grassed slopes under simulated rainfall

It is still unclear how slope steepness (S) and revegetation affect resistance (f) to overland flow. A series of experiments on runoff hydraulics was conducted on granular surfaces (bare soil and sandpaper) and grassed surfaces, including grass plots (GP), GP with litter (GL), and GP without leaves (GS) under simulated rainfall and inflow (30<Re<1400) with varying slopes ranging from 2.6% to 50%. The results show that the observed f based on a small-size runoff plot under rainfall conditions tends to be overestimated due to the increase in flow rate, or Re (Reynolds number), with downward cross sections and a good f-Re relation (f = KRe−1). There exists a good f-Re relation for granular surfaces and a good f-Fr relation (Fr, Froude number) for grass plots. A greater f occurred at the gentle and steep slopes for the granular surfaces, while f decreased with increasing slopes for the grass treatments. The different f-S relations suggest that f is not a simple function of S. When Re≈1000, the sowing rye grass with level lines increased f by approximately 100 times and decreased bed shear stress to approximately 5%. The contribution of grass leaves, stems, litter, and grain surface to total resistance in the grass plots were averagely 52%, 32%, 16%, and 1%. The greater resistance from leaves may result from the leaves lying at the plot surface impacted by raindrop impact. These results are beneficial to understand the dynamics of runoff and erosion on hillslopes impacted by vegetation restoration.

Particle selectivity of sediment deposited over grass barriers and the effect of rainfall

Particle selectivity of the sediment deposited over vegetative barriers is of importance to predict sediment transport and particulate pollutant load into surface waters. Grassed barriers with 20%, 40%, 60%, 70% and 90% covers at 15° slope were subjected to silt-laden inflows in the presence and absence of simulated rainfalls to investigate the sediment deposition processes. The results show that re-grass of steep croplands can effectively trap eroded sediment from upslope, and the rowed grass barriers can strengthen sediment deposition. The deposition order of sediment particle sizes (μm) follows (>50)> (25-50)>(10-25)=( (2-10), and the particle selectivity weakens with increasing grass covers. Clay particles had a similar deposition efficiency to overall sediment, implying the effectiveness of re-grass in controlling soil nutrient loss. The contribution of grass to total overland flow resistance is almost equivalent to the percentage of grass cover. For steep grassed slopes, raindrop impact significantly decreases sediment deposition, but limitedly affects particle selectivity of deposited sediment and overland flow hydraulics. Both raindrop kinetic energy and stream power available for surface soil contribute to sediment deposition in net deposition areas of grass barriers. These imply that rainfall effect on sediment delivery over vegetated barriers derives from the additional raindrop energy, rather than the variation in runoff hydraulics. These results can help to clarify the effect of raindrop impact on sediment transport and to evaluate the benefit of re-vegetation in decreasing sediment yield and its particulate nutrient load into surface waters. This article is protected by copyright. All rights reserved.